Pull a wire from a plug, put a new plug on the wire and have someone turn the ignition. Look at the spark at the plug. If it gives a blueish red spark, it is probably time for a new coil. The spark is supposed to be white and bright and you supposed to hear the spark. I'm not an expert, so if someone has any other ideas on how to check the coils, I would like to know also.
About three years ago, my 90 300E would not start. I play with it so much that I flooded the intake manifold with gas. I messed up the lambda adjustment so bad that became impossible for me to start the car. Finally I towed it to Mercedes, cost me a bundle to troubleshoot, because of my fixing
and turned out was the coil.
Lately I had similar problems with the 92 300TE. At times was hard to start and other times would not start at all. Checked the spark and noticed that when the spark was bright the car started. When the spark was dim the car would not start. I believe the inconsistencies was from the distributor cap, rotor and coil. Changed it all and the car know starts normal.
Good luck
Michael